Dielectrically impeded discharge lamps are known per se and are largely distinguished by the fact that at least some of the electrodes used for igniting and maintaining the discharge are separated from the discharge medium by a dielectric layer. They are generally also known as “silent discharge lamps”. Such discharge lamps are started and operated using electronic ballasts or, more generally, operating circuits. Higher voltages, and thus higher amplitudes when power is input, are generally required for ignition purposes than during continuous operation.
Operating circuits for such lamps generally contain a converter for inputting the power to the lamp. In principle, such discharge lamps having a very varied AC voltage rating are to be operated, in particular a pulsed mode of operation having power-input phases which are temporally separated by power-input-free times being of interest owing to the increases in efficiency achieved thereby. However, the invention relates in principle to operating circuits of any type for dielectrically impeded discharge lamps. It is known to connect a switching transistor, which is responsible, owing to its switching operation, for the ignition process and, in the event of a pulsed continuous power input, also for the actual lamp operation, in a line path supplying the converter with current. The converters used generally have an inductive characteristic; specifically they are generally transformers having a primary winding which has current applied to it by the abovementioned switching transistor.
It is likewise already known to protect this switching transistor for the event in which an attempt is made to start operation without a lamp having been correctly connected. In this case, energy is built up in the inductance of the converter, i.e. in the primary winding of a transformer, for example, and this energy, when it is not at least partially consumed by the lamp, is dissipated in the switching transistor. An overvoltage protection circuit may be used which measures the voltage across this transistor, for example the drain-source voltage in the case of a FET, and in the event of a threshold value being exceeded, ends the lamp operation.